The present invention relates to curable silicone rubber compositions. More particularly, the present invention relates to one-component, alkoxy-functional, room temperature vulcanizable (RTV) silicone compositions which exhibit low modulus upon curing. Processes for making such low modulus silicone compositions are also provided.
Elastomeric siloxane compositions capable of vulcanizing at room temperature have achieved considerable commercial success. In certain applications, such as high rise building construction, it is desirable and often critical to utilize low modulus sealants for adhering the window panes to the metal frame of the building. This low modulus property enables the silicone elastomer to easily compress and expand with building movement due to winds and the like without causing cohesive or adhesive failure. In addition to being a low modulus composition, it is often important that the silicone composition also have high tensile strength and high elongation. These properties will ensure that the silicone elastomer can accommodate, for example, plus or minus 50% compression or extension in the joints without failing. Of course, the more the sealant can expand and compress in terms of thickness of the joint, the more desirable it is for use as a construction sealant.
Because of the recent emphasis by architects on "mirrored" high rise buildings, that is, the exterior of the building appears to be a large mirror, for both aesthetic and energy-saving reasons, there has been a great deal of interest in providing suitable low modulus silicone sealants. Perhaps the most common type of low modulus silicone sealant is based on a silanol endblocked polydiorganosiloxane in combination with any of a number of crosslinking and/or coupling agents which impart low modulus to the cured composition.
Clark et al., U.S. Pat. No. 3,766,127, discloses a composition stable under moisture free conditions and curable to a low modulus silicone elastomer comprising a mixture of 100 parts by weight of a hydroxyl endblocked polydiorganosiloxane; 0 to 150 parts by weight of a non-acidic, non-reinforcing filler; 3.5 to 8 parts by weight of ##STR1## in which R is methyl or vinyl, R' is methyl, ethyl or phenyl and 0.3 to 4.2 parts by weight of ##STR2## in which R' is defined above and R" is the same as R.
Brady et al., U.S. Pat. No. 3,766,128, relates to a low modulus room temperature vulcanizable silicone elastomer obtained by curing a mixture of 100 parts by weight of a hydroxyl endblocked polydiorganosiloxane; 0 to 150 parts by weight of a non-acidic, non-reinforcing filler; 2 to 7.5 parts by weight methylvinyldi-(N-methylacetamido)silane and 0.5 to 4 parts by weight of an aminosilane of the formula RSi(NR'R").sub.3 in which R is alkyl, phenyl or vinyl, R' is hydrogen or alkyl and R" is alkyl or phenyl.
Toporcer et al., U.S. Pat. No. 3,817,909, describes low modulus room temperature vulcanizable silicone elastomers obtained by mixing 100 parts by weight of a hydroxyl endblocked polydiorganosiloxane; 0 to 150 parts by weight of a non-acidic, non-reinforcing filler; 2 to 20 parts by weight of ##STR3## in which R is methyl, vinyl or phenyl and R' is methyl, ethyl or phenyl and 0.25 to 7 parts by weight of an aminooxysilicon compound having 3 to 10 aminoxy groups per molecule.
Klosowski, U.S. Pat. No. 3,996,184, relates to a one package, low modulus, room temperature vulcanizable composition obtained by mixing 100 parts by weight of a hydroxyl endblocked polydimethylsiloxane; 0 to 200 parts by weight of a non-acidic, non-reinforcing filler; 2.5 to 10 parts by weight of ##STR4## in which R is methyl or phenyl; 1 to 6 parts by weight of an aminoxysilicon compound having 3 to 10 aminoxy groups per molecule; and 1 to 5 parts by weight of N,N-dimethylformamide, acetonitrile, or N-n-butylacetamide.
Beers, U.S. Pat. No. 4,100,129, discloses self-bonding, low modulus, one-package room temperature vulcanizable silicone compositions comprising a silanol chainstopped polydiorganosiloxane; a crosslinking silane; and a silanol reactive organometallic ester compound of a metal, the compound having radicals attached to the metal atom, at least one of the radicals being a substituted or unsubstituted hydrocarbonoxy radical, attachment being through an M--O--C linkage where M is the metal, and any remaining valences of M are satisfied by other organo radicals attached to M through such linkages, or an --OH, --O--, or M--O--M linkage, the weight ratio of the organometallic ester to silane being at least 0.5 to 1.
Beers, U.S. Pat. No. 4,323,489, teaches a room temperature vulcanizable silicone rubber composition with low modulus comprising a silanol endstopped diorganopolysiloxane, a difunctional acetamide coupler, and as the crosslinking agent, a minor amount of compound selected from ketoxime functional and aminoxy functional silanes and mixtures thereof.
Hahn, U.S. Pat. No. 4,360,631, describes a flowable, low modulus, room temperature vulcanizable silicone composition obtained by mixing 100 parts by weight of hydroxyl endblocked polydiorganosiloxane; 0 to 150 parts by weight of a non-acidic, non-reinforcing filler; 2 to 20 parts by weight of methylvinyldi-(epsilon-caprolactamo)silane and 0.25 to 7 parts by weight of an aminoxysilane compound having 3 to 10 aminoxy groups per molecule.
Lampe, U.S. Pat. No. 4,410,677, provides a low modulus, room temperature vulcanizable silicone rubber composition with a good shelf life comprising a silanol containing polysiloxane, a filler, an acyloxy functional silane as the crosslinking agent, and as the catalyst, a compound selected from the group consisting of zinc salts and zirconium salts, or a co-catalyst system comprising as one co-catalyst a tin salt of a carboxylic acid and as the other co-catalyst, either a zinc salt or a zirconium salt of a carboxylic acid.
Another class of low modulus silicone compositions is based on alkoxy terminated polydiorganosiloxanes originally disclosed by Brown et al. in U.S. Pat. No. 3,161,614. The problem with providing low modulus RTV compositions based on polyalkoxy terminated siloxanes is that they already contain at least two terminal functional groups for effecting crosslinking whereas silanol terminated siloxanes are monofunctional and can easily be chain-extended with a coupling agent to provide the desired low modulus.
Cooper et al., U.S. Pat. No. 3,542,901, teaches that one-component silicone RTV compositions of the type disclosed by Brown et al. are ineffective for certain sealing applications because the modulus is too high. Cooper et al. goes on to disclose a method to provide compositions vulcanizable to low modulus rubbers comprising utilizing a polydiorganosiloxane having therein a proportion, up to 40 percent, of terminal units of the formula --SiR.sub.3, the remaining terminal units being alkoxy functional. Preferably the polydiorganosiloxane has a viscosity within the range of 2,000 to 20,000 centipoise at 25.degree. C.
Beers, U.S. patent application Ser. No. 349,537, filed Feb. 17, 1982, now abandoned, discloses that alkoxy functional RTV compositions of the type disclosed by White et al., U.S. Pat. No. 4,395,526, and Dziark, U.S. Pat. No. 4,417,042, can be made low modulus by adding thereto from 2 to 20 parts by weight of a plasticizer fluid containing a high degree of trifunctional siloxy units or a mixture of trifunctional and tetrafunctional siloxy units. Optionally, additional plasticizers such as linear triorganosiloxy endstopped polydiorganosiloxanes having a viscosity ranging from 10 centipoise to 20,000 centipoise can be included in the composition.
Other patents of interest with respect to one or more aspects of the present invention are Murphy, U.S. Pat. Nos. 3,341,486, 3,441,583, 3,528,941 and 3,926,199 which are directed to aminoxy functional RTV compositions. Golitz et al., U.S. Pat. No. 3,417,042 provides an acetamide or amide functional one-component RTV composition formed by mixing a silanol terminated base polymer with an amide functional silane or siloxane crosslinking agent. A similar composition is disclosed in Klebe, U.S. Pat. No. 3,488,371 as well as in Toporcer, U.S. Pat. No. 3,776,934.
All of the foregoing patents and patent applications are incorporated by reference into the present patent application.
Although a wide choice is thus seen to be available for the preparation of low modulus, one-component RTV compositions, there has not been much appreciation by those skilled in the art of the factors involved in securing specific properties in the cured silicone rubber, especially when the polysiloxane is alkoxy functional. It is known, for example, that some variations in properties can be induced in the cured composition by varying the molecular weight of the polydiorganosiloxane. That is, as the molecular weight increases, elongation of the rubber also increases and, on the other hand, if a lower viscosity material is used, the cure is tighter causing lower elongation and increased hardness. The adhesion of the composition to a variety of conventional substrates is also a factor which can generally be predictably influenced, for example, by incorporating a suitable adhesion promoter. In any event, both modulus control and adhesion control are not easily achieved, especially in polyalkoxy functional systems, and there still is a lot to be desired in the present state of the art. Of particular concern is the need to include plasticizing fluids in alkoxy functional one component silicone RTV compositions in order to obtain commercially useful low modulus compositions. The problem with such compositions is that volatiles in the plasticizing fluids tend to migrate out of the sealant and onto the sealant substrate. Subsequently, dirt and dust particles become embodied therein, adversely affecting the substrates appearance by causing it to appear "stained".
It has now been discovered that alkoxy functional, low modulus, silicone RTV compositions can be prepared from conventional ingredients at properly selected ratios without the need for such plasticizing fluids.